The invention is based on an illumination unit having at least one LED as light source in accordance with the preamble of claim 1. This is in particular an LED which emits in the visible or white region and is based on an LED which emits primarily UV/blue.
An illumination unit which emits, for example, white light is currently obtained predominantly by combining a Ga(In)N-LED, which emits in the blue at approximately 460 nm, and a yellow-emitting YAG:Ce3+ phosphor (U.S. Pat. No. 5,998,925 and WO 98/12757). For good color rendering, two different yellow phosphors are often used, as described in WO-A 01/08453. A problem in this case is that the two phosphors often have different temperature characteristics, even if their structures are similar. A known example is the yellow-luminescent Ce-doped Y garnet (YAG:Ce) and the (Y,Gd) garnet which, by comparison, is luminescent at a longer wavelength. This leads to fluctuations in the color locus and changes in the color rendering at different operating temperatures.
The publication xe2x80x9cOn new rare-earth doped M-Sixe2x80x94Alxe2x80x94Oxe2x80x94N materialsxe2x80x9d by van Krevel, TU Eindhoven 2000, ISBN 90-386-2711-4, Chapter 11 has disclosed a number of classes of phosphor materials which have the structure of nitrides or oxynitrides or are known as sialons (in particular (xcex1-sialons), which represent a contraction of their composition. An emission in a wide optical spectral region with excitation at 365 nm or 254 nm is achieved by means of doping with Eu, Tb or Ce.
It is an object of the present invention to provide an illumination unit having at least one LED as light source,the LED emitting primary radiation in the range from 300 to 570 nm, this radiation being partially or completely converted into the longer-wave radiation by phosphors which are exposed to the primary radiation of the LED and the structure of which is based on nitrides or derivatives thereof, which is distinguished by a high level of constancy at fluctuating operating temperatures. A further object is to provide an illumination unit which emits white light and in particular has a good color rendering and a high output. A further object is to provide a new daylight fluorescent pigment.
This object is achieved by the following features: the conversion takes place with the aid of at least one phosphor which is derived from a cation M and a silicon nitride or a derivative of a nitride which emits with a peak emission wavelength at 430 to 670 nm, the cation being partially replaced by a dopant D, namely Eu2+ or Ce3+, at least one of the divalent metals Ba, Ca, Sr and/or at least one of the trivalent metals Lu, La, Gd, Y being used as cation M, the phosphor originating from one of the following classes which provide as such new daylight fluorescent pigments:
nitrides of the structure MSi3N5, M2Si4N7, M4Si6N11 and M9Si11N23,
oxynitrides of the structure M16Si15O6N32 
sialons of the structure MSiAl2O3N2, M13Si18Al12O18N36, MSi5Al2ON9 and M3Si5AlON10.
Particularly advantageous configurations are given in the dependent claims.
According to the invention, the phosphor used for the LEDs is a phosphor from one of a number of nitride-based phosphor classes.
These are certain classes of nitrides and their derivatives oxynitrides and sialons. The phosphor which is derived from a cation M and a silicon nitride or a derivative of a nitride emits with a peak emission wavelength of 430 to 670 nm, the cation being partially replaced by a dopant D, namely Eu2+ or Ce3+, the cation M used being at least one of the divalent metals Ba, Ca, Sr and/or at least one of the trivalent metals Lu, La, Gd, Y, the phosphor originating from one of the following classes:
nitrides of the structure MSi3N5, M2Si4N7, M4Si6N11, and M9Si11N23,
oxynitrides of the structure M16Si15O6N32;
sialons of the structure MSiAl2O3N2, M13Si18Al12O18N36, MSi5Al2ON9 and M3Si5AlON10.
The following specific phosphors are particularly preferred:
Mxe2x80x2Mxe2x80x3Si4N7:Dxe2x80x83xe2x80x831.
where
Mxe2x80x2=Sr or Ba, in each case alone or in combination, and in particular Mxe2x80x2 is partially (up to 20 mol %) replaced by Ca; Mxe2x80x2 is a divalent ion.
Mxe2x80x3=Lu alone or in combination with Gd and/or La; Mxe2x80x3 is a trivalent ion.
A specific example is SrLuSi4N7:Eu2+.
Mxe2x80x2Mxe2x80x3Si6N11:Dxe2x80x83xe2x80x832.
where Mxe2x80x2=BaxSr3xe2x88x92x, preferably x=1.5; Mxe2x80x2 is divalent;
where Mxe2x80x3=Lu alone or in combination with Gd and/or La and/or Y; Mxe2x80x3 is trivalent;
To a certain extent, the quantities of Ba2+ and Sr2+ may vary (the value for x may fluctuate between 1.3 and 1.7), and these components may be partially (up to 20 mol % of the total quantity of Mxe2x80x2) replaced by Ca2+.
A specific example is BaLuSi6N11:Eu.
Mxe2x80x33Si6N11:Dxe2x80x83xe2x80x833.
Where Mxe2x80x3=La alone or in combination with Gd and/or Y and/or Lu; Mxe2x80x3 is a trivalent ion.
Preferably, D=Ce3+.
A specific example is La3Si6N11:Ce.
Mxe2x80x22Mxe2x80x37Si11N23:Dxe2x80x83xe2x80x834.
Where Mxe2x80x2=Ba alone or in combination with Sr (up to 50 mol %)
Mxe2x80x3=La alone or in combination with Gd and/or Lu;
A specific example is Ba2La7Si11N23:Eu
Mxe2x80x3Si3N5:Dxe2x80x83xe2x80x835.
Where Mxe2x80x3=La alone or in combination with Gd and/or Lu;
Where D=Ce.
A specific example is LaSi3N5:Ce.
Furthermore, they may be certain classes of oxynitrides, namely those of type Mxe2x80x316Si15O6N32:D. As trivalent cation Mxe2x80x3, these oxynitrides use at least one of the metals La, Gd, Lu or Y. The cation is partially replaced by a dopant D, namely Eu2+ or Ce3+. The following specific phosphors are particularly preferred:
Mxe2x80x316Si15O6N32:Cexe2x80x83xe2x80x836.
where Mxe2x80x3=La alone or in combination with Gd and/or Lu;
a specific example is La16Si15O6N32:Ce.
Furthermore, they are certain classes of sialons, i.e. those of type MSiAlON:D. As divalent or trivalent cation Mxe2x80x3, these sialons use at least one of the metals Ba, Sr, Ca, La, Gd, Lu or Y. The cation is partially replaced by a dopant D, namely Eu2+ or Ce3+. The following specific phosphors are particularly preferred:
Mxe2x80x2SiAl2O3N2:Dxe2x80x83xe2x80x837.
where Mxe2x80x2=Sr alone or in combination with Ba and/or Ca2+; the proportion of Ba may be up to 50 mol %, and the proportion of Ca may be up to 20 mol %.
A specific example is SrSiAl2O3N2:Eu.
Mxe2x80x23Mxe2x80x310Si18Al12O18N36:Dxe2x80x83xe2x80x838.
where Mxe2x80x2=Sr alone or in combination with Ba and/or Ca; the proportion of Ba may be up to 50 mol %, and the proportion of Ca may be up to 20 mol %;
where Mxe2x80x3=La alone or in combination with Gd and/or Lu;
preferably, Mxe2x80x2=Sr2+ and/or Mxe2x80x3=La3+;
a specific example is Sr3La10Si18Al12O18N36:Eu.
Mxe2x80x3Si5Al2ON9:Ce3+xe2x80x83xe2x80x839.
where Mxe2x80x3=La alone or in combination with Gd and/or Lu;
A specific example is LaAl2Si5ON9:Ce.
Mxe2x80x33Si5AlON10:Ce3+xe2x80x83xe2x80x8310.
where Mxe2x80x3=La alone or in combination with Gd and/or Lu;
Preferably, Mxe2x80x3=La3+.
A specific example is La3Si5AlON10:Ce.
The proportion of the dopant (i.e. the Eu or Ce content) which replaces some of the cation M should be 0.5 to 15%, preferably 1 to 10%, of the M cation, so that the emission wavelength can be selected accurately and the light efficiency can be optimized. An increasing dopant content generally shifts the peak emission toward longer wavelengths. Surprisingly, it has been found that a changing concentration of the cation M also shifts the peak emission wavelength. At a low concentration of the M cation, good absorption by the dopant can be obtained by selecting the amount of this dopant to be 5 to 10 mol % of the M cation.
These novel optically active materials can be summarized as pigments with daylight fluorescence, in particular also as phosphors. What this means is that the material can be used either as a pigment or as a light-converting system for applications such as displays, lamps or LEDs, or may even be suitable for both purposes.
A further promising representative of this class of the Eu-activated sialons is an xcex1-sialon which corresponds to the formula Mp/2Si12xe2x88x92pxe2x88x92qAlp+qOqN16xe2x88x92q:Eu2+, where M=Ca individually or in combination with at least one of the metals Sr or Mg, where q=0 to 2.5 and p=0.5 to 3, referred to below as a YO sialon.
These novel optically active materials are preferably doped with (or contain) M2+=Eu2+ or M3+=Ce3+. Moreover, in the case of Ce-doping, there may also be a small amount of co-doping (up to 30 mol % of the Ce) with Pr3+ or Tb3+. In the case of doping with Eu, co-doping (up to four times the amount of Eu) with Mn2+ is possible. With these combinations, it is possible for energy to be transferred from the first doping to the co-doping.
With regard to the application as a means for converting radiation sources with primary radiation between 300 and 570 nm, optically active materials doped with Eu are particularly preferred.
The novel optically active materials are all very robust and are also thermally and chemically stable, since their basic structure is based on tetrahedra, either of the Sixe2x80x94(O,N) or Alxe2x80x94(O,N) type. In this context, the term Sixe2x80x94(O,N) or Alxe2x80x94(O,N) tetrahedron means: firstly one of the groups SiN4, SiON3, SiO2N2 or SiO3N, and secondly one of the groups AlN4, AlON3, AlO2N2 or AlO3N. The materials whose basic structure includes Si and/or Al tetrahedra with at least two or more nitride (N3) ligands are preferred. Generally, it has been established that the absorption of optically active ions D (irrespective of whether they are divalent or trivalent) which have a broadband absorption shifts toward longer wavelengths as the proportion of N in the tetrahedra rises.
The absorption of divalent activators D2+, preferably Eu2+ on its own, may in principle be shifted from the UV into the orange-red (up to approximately 590 nm) , depending on the amount of nitride in the tetrahedra. The absorption of trivalent activators D3+, preferably Ce3+ on its own, can in principle be shifted from the UV into the blue-green (up to approximately 495 nm) , depending on the amount of nitride in the tetrahedra. Further factors which influence the position of the absorption maximum are the coordination and the specific lattice point at which the activator ion is located.
The preferred lattice positions for D2+ are Mxe2x80x2=Sr2+ and Ca2+, but Ba2+ is also suitable. Coordination numbers from 6 to 9 are preferred with regard to these divalent cations. The lower the coordination number, the longer the absorption wavelength. The coordination number is dependent on the volume considered, i.e. the greater the volume selected, the higher the coordination becomes. For example, in SrSiAl2O3N2, the ion Sr2+ is coordinated by ligands in the form of the anions N3xe2x88x92 and O2xe2x88x92. Specifically, there are six ligands with a spacing for Sr2+ of 2.04-2.95 xc3x85, and furthermore two additional ligands with a spacing of approximately 3.04 xc3x85, and finally also one further ligand with a spacing of 3.18 xc3x85. Therefore, depending on the volume considered, the coordination number is either 6, 8 or 9.
Table 1 below presents the preferred maximum spacings of the coordinated ions, in each case using the mean of the spacings of all the closest ions taken into consideration in connection with the coordination. This applies in the case of exclusively divalent cations Mxe2x80x2 or at least predominantly (more than 80%) divalent cations Mxe2x80x2. By way of example, Table 1 shows the following: an Eu2+ ion, for example at a Ba2+ location in a lattice, should have seven ligands with a mean spacing of at most 3.015 xc3x85, or should have eight ligands with a mean spacing of at most 3.02 xc3x85. In each case one of these conditions, in particular the condition for the lowest ligand number, should be satisfied in order to achieve the desired good properties of the pigment. The ions Ba2+ and Sr2+ are so large that in general they always have at least six ligands gathered around them. The smaller ion Ca2+ in some cases makes do with five ligands. In the case of mixed compounds of the three cations Mxe2x80x2, the condition relating to the dominant cation applies.
For optical applications, in which D2+=Eu2+, and in which the optically active material is intended to convert light with wavelengths of between 300-570 nm partially or completely into visible light, the preferred ions are Sr2+ and Ca2+. The condition which is preferably to be observed with regard to the coordination is, for Sr2+, the condition for six or seven ligands. The condition with regard to the coordination which is preferably to be observed for Ca2+ is the condition for five or six ligands.
Compounds which satisfy at least one of the conditions in Table 1 have a high absorption, with a maximum between 300 and 570 nm, and convert efficiently.
They are in particular compounds from class 7 (Mxe2x80x2SiAl2O3N2:D) and the xcex1-sialons as described in German application 101 33 352.8. Table 2 gives a number of examples.
Table 2: spacings A1 to A7 (in xc3x85) of the first to seventh closest ligand, and mean Mw5 to Mw7 calculated therefrom for the spacings of the first five to seven ligands, based on the Ca or Sr ion, for various compounds.
Compounds of this type are thermally and chemically stable. For applications in which these optically active materials have to be dispersed (for example in casting resin of an LED), a further advantage of these materials is that they have a good impact strength and are damaged scarcely, if at all, during the milling process carried out in mills. Such damage to the grains as a result of the milling operation reduces the efficiency of other phosphors.
Material design methods make it possible to deliberately create phosphors which are based on Si/Alxe2x80x94N and have a specific emission in a wide range between blue and dark red.
A particular advantage of these nitride-based systems is that it thereby becomes possible for a plurality of Si/Alxe2x80x94N-based phosphors with similar physical characteristics to be used together in order, for example, to produce a white LED. A similar consideration also applies with regard to the primary light source, which is very often also nitride-based, since it generally involves semiconductor components based on InN, GaN and AlN. The Si/Alxe2x80x94N-based phosphors according to the invention can in this case be applied directly with particular success.
Particular advantages of these phosphors in connection with an LED-based illumination unit are a high efficiency, excellent thermal stability (insensitive to changes in the operating temperature) and a surprisingly high luminescence extinction temperature, as well as the associated high color rendering, in particular in combination with at least one further phosphor.
A further advantage of this class of phosphors is that the starting material (in particular Si3N4) is already present in extremely finely dispersed form. Consequently, it is often not necessary to mill the phosphor. By contrast, conventional phosphors, such as YAG:Ce, which are likewise produced by solid-state synthesis, have to be milled, so that they remain dispersed in the casting resin and do not sink to the bottom. This milling operation often leads to efficiency losses. These phosphors therefore no longer have to be milled, which saves one operation and means that there are no efficiency losses. Typical mean grain sizes of the phosphor powder are 0.5 to 5 xcexcm.
In addition to the production of a colored light source by excitation by means of UV radiation from an LED, in particular the generation of white light with the aid of these phosphors offers advantages. This is achieved with a UV-emitting LED as primary light source by using at least three phosphors, and with a blue-emitting LED as primary light source by using at least two phosphors.
White light with good color rendering is generated in particular by the combination of a UV-LED (e.g. primary emission at 300 to 470 nm) with two to three phosphors, of which at least one is a nitride-containing phosphor according to the invention.
The major advantages of nitride-containing phosphors are their pronounced stability with respect to hot acids, lyes and also their thermal and mechanical stability.